Bryne Formation (new)
(From NPD Bulletin no. 3)
Recommended as Member in Norlex
Vestland Group
Name
From a town in the south-western part of Norway. This new formation
represents the lower part of the Haldager Formation as described by Deegan and Scull (1977).
Well type section
Norwegian well 9/4-3
(Conoco) from 2507.5 m to 2613 m, coord N 57°36'54.5",
E 04°18'57.7" (Fig. 29).
Well reference section
Norwegian well 8/12-1
(Conoco) from 2710.5 m to 2813 m, coord N57°13'18.6", E03°46'45.13" (Fig. 30).
Thickness
The formation is 105.5 m thick in the type well and 102.5 m in the reference well. It shows local variations in thickness which probably reflect both Middle Jurassic syndepositional structuring and later erosion.
Lithology
The Bryne Formation comprises interbedded sandstones, siltstones, shales and coals. The sandstones are white to grey, very fine to coarse grained, poorly sorted, friable to hard and occasionally kaolinitic. The shales are generally grey to brown, micaceous, occasionally silty, non-calcareous and often carbonaceous.
Boundaries
The base of the Bryne Formation is unconformable and represents the contact
with the partly eroded shales of the Fjerritslev
Formation or with arenaceous Triassic rocks. The boundary with the
Fjerritslev Formation is usually clearly defined on both gamma ray and
sonic logs, whereas the boundary with the Lower Jurassic/Triassic sandy
sequences (Gassum and Skagerak Formation) often gives
no appreciable log breaks. However, on most logs the appearance of these
sediments is marked by an overall sonic log shift to higher interval
velocities. Where the formation is overlain by the shales of the
Boknfjord or Tyne Groups,
clear breaks can be observed both on gamma ray and sonic logs. However, where
the formation is overlain by the Sandnes or
Ula Formation the boundary is not so easily defined due
to internal facies changes within these two formations (see description of
the Sandnes and Ula Formation).
Distribution
The Bryne Formation is present in the Norwegian-Danish Basin and in the Central Graben. Two main Middle Jurassic depocentres are recognized; one in the Danish Sub-Basin and another in the Fiskebank Sub-Basin (Hamar et al., 1982).
The Bryne Formation is equivalent to the Haldager Sand Member of the Haldager Formation as described in Denmark (Larsen, 1966; Michelsen, 1978).
Age
Mainly Bajocian to Bathonian, but may locally
be older in the Norwegian-Danish Basin.
Depositional environment
The Bryne Formation represents deposition in a
fluvial/deltaic environment.
Remarks
The Bryne Formation as described above is approximately equivalent in age
and lithofacies to the Sleipner Formation of the
Southern Viking Graben. So far it is not possible to demonstrate a connection
between the two deposits, and this constitutes the reason for use of separate
nomenclature.
The Bryne Formation represents the lower part of the Haldager Formation,
extended into the Norwegian sector from the Danish sector by Deegan and
Scull (1977). Having defined extensive marine sands worthy of formation
status (Ula, Sandnes Formation)
comprising the upper part of Deegan and Scull's Haldager Formation, workers on this project saw the need for a separate formation, defining the lower non-marine sands. The term "Haldager Formation" could not be used since it essentially equates to the Haldager Sand Member, the lower part of the Haldager Formation in the Danish sector (Michelsen, 1978). (The upper part of the Danish Haldager Formation is the Flyvberg Member, a marine sandstone/siltstone unit approximately time-equivalent to the Sandnes Formation of the Norwegian-Danish Basin). When a previously established formation is subdivided into new units which are formally given formation status, the original formation with its original name should be either raised to group rank or abandoned; the old name should not be retained for any of the divisions of the original unit (Hedberg, 1976).It was therefore though expedient to abandon the name "Haldager Formation" in the Norwegian sector. It is recognised, however, that there is probably complete lithological continuity between the new Bryne Formation of the Norwegian sector and the Haldager Sand Member of the Danish Haldager Formation.
